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Clinical Chemistry Lecture 3 > Endocrinology > Flashcards

Endocrinology Flashcards

0
Q

Hypothalamic-pituitary-thyroid axis and thyroid hormone synthesis

A

(Review)

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1
Q

Weight of thyroid gland

A

15 to 25 grams

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2
Q

Composition of lobules

A

20-40 follicles separated by highly vascular connective tissue

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3
Q

Surrounds a closed cavity containing colloid, thyroid hormone, thyroglobulin, and a variety of glycoproteins

A

Follicular cells

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4
Q

Responsible for the synthesis and secretion of calcitonin, a hormone important in calcium metabolism

A

Parafollicular cells or C cells

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5
Q

Hormone stimulated by hypothalamus

A

TRH

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6
Q

Responsible for release of TSH

A

Anterior pituitary gland

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7
Q

Thyroid hormone synthesis

A

1 Active transport of inorganic iodide into the cell (follicular cell)
2 Iodination of the tyrosyl residues on Thyroglobulin (Tg)
3 Coupling of iodotyrosine molecules within Tg to form T4 and T3 (two separate oxidative reactions, both catalyzed by thyroid peroxidase, TPO)
T3 (1 DIT, 1 MIT)
T4 (2 DIT)

4 Proteolysis of Tg with release of free iodotyrosine, T4, and T3, and secretion of iodothyronine into the circulation
5 Deiodination of iodotyrosines within the thyroid and reuse of liberated iodide
6 Back to 1
7 Release of thyroid hormones

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8
Q

Thyroidal origin:
T4
T3

A

100%

20%

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9
Q

T3: 80%

A

Produced enzymatically in nonthyroidal tissues by 5’-monodeiodination of T4

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10
Q

T4

A

1 Thyroxine-binding globulin (TBG): 70%
2 Transthyretin: 20%
3 Albumin: 10%

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11
Q

Other term for transthyretin

A

Binding prealbumin

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12
Q

Free thyroid hormones

A

Metabolically active

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13
Q

T or F. Most of the circulating T3 is bound to TBG with a 10-fold reduced affinity as compared with that of T4

A

T

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14
Q

Third major form of circulating hormone

A

rT3

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15
Q

Reverse T3: Characteristics

A

1 No biological activity
2 Short half-life of 4 hours
3 Circulates bound to TBG
4 Its formation is considered a disposal pathway in the peripheral metabolism of T4

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16
Q

40% undergoes deiodination of the inner ring of T4 to form rT3

A

Monoiodinated T4

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17
Q

Influenced by intake of iodine

A

MIT/DIT Ratio

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18
Q

Preferential iodotyrosine formed

A

DIT

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19
Q

Predominant form of hormone synthesized and secreted when iodide is abundant

A

T4

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20
Q

Produced in greater quantities when iodide sources are diminished

A

MIT

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21
Q

Result of MIT production in great quantities

A

Increased T3 formation and release

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22
Q

Most common clinical syndrome caused by circulating antibodies to the TSH receptor

A

Graves’ disease

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23
Q

Signs and symptoms of hyperthyroidism

A

Heat intolerance, tachycardia, weight loss, weakness, emotional lability, tremor, diarrhea

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24
Q

Opposite of myxedema

A

Thyroid storm

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25
Q

Myxedema coma

A

Advanced stage of thyroid hormone deficiency characterized by progressive stupor, hypothermia, and hypoventilation

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26
Q

Hypothyroidism signs and symptoms

A

Hoarseness, cold sensitivity, dry skin, constipation, bradycardia, muscle weakness

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27
Q

Hypothyroidism that involves the thyroid gland

A

Primary

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28
Q

Hypothyroidism that involves the pituitary gland

A

Secondary

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29
Q

Hypothyroidism that involves the hypothalamus

A

Tertiary

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30
Q

Secondary hypothyroidism

A

TSH secretion is decreased as a result of a pituitary disorder

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31
Q

Tertiary hypothyroidism

A

Hypothalamic dysfunction

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32
Q

Causes of primary hypothyroidism

A

1 Ablation with radioactive iodine

2 Surgery to treat hyperthyroidism

33
Q

Primary hypothyroidism

A

Most commonly iatrogenic in origin where there is failure to secrete thyroid hormones

34
Q

Examples of euthyroidism

A

1 Goiter
2 Thyroid adenoma
3 Thyroid carcinoma

35
Q

Permit the diagnosis of subclinical hyperthyroidism and hypothyroidism

A

Accurate TSH assays

36
Q

History: tremors, profuse sweating, nodule in the thyroid gland
Tests of T4 and T3: normal
TSH suppressed; T4 and T3 overproduced

A

Euthyroidism

37
Q

Other term for T3

A

3,5,3-triiodothyronine

38
Q

Other term for T4

39
Q

T or F. 80% of circulating T3 comes from the peripheral deiodination of T4

40
Q

Production rate, nmol/day
T4
T3

41
Q

Fraction of circulating hormone of thyroid origin
T4
T3

42
Q

Fraction of total hormone in free form
T4
T3

A
  1. 0002

0. 003

43
Q

Half life, days
T4
T3

44
Q

Relative metabolic potency
T4
T3

45
Q

Serum concentration, Total, nmol/L
T4
T3

46
Q

Serum concentration, Free, pmol/L
T4
T3

47
Q

Divide by 12.87

A

Conversion of
1 Total T4 from nmol/L to mcg/dL
2 Free T4 from pmol/L to ng/dL

48
Q

Multiply by 65.1

A

Conversion of
1 Total T3 from nmol/L to ng/dL
2 Free T3 from pmol/L to pg/dL

49
Q

Provides an accurate diagnostic measurement

A

Most of ingested iodine excreted in urine

50
Q

Used mainly to assess dietary intake of populations

A

Urinary iodine measurement

51
Q

Degree of deficiency: none

A

Urinary iodine excretion, mcg/L: > 100

52
Q

Degree of deficiency: none

A

Goiter prevalence: <5%

53
Q

Degree of deficiency: mild

A

Goiter prevalence: 5%-19.9%

54
Q

Degree of deficiency: mild

A

Urinary iodine excretion, mcg/L: 50-99

55
Q

Degree of deficiency: moderate

A

Goiter prevalence: 2%-29.9%

56
Q

Degree of deficiency: moderate

A

Urinary iodine excretion, mcg/L: 20-49

57
Q

Degree of deficiency: severe

A

Goiter prevalence: >30%

58
Q

Degree of deficiency: severe

A

Urinary iodine excretion, mcg/L: <20

59
Q

Specimens for neonatal hypothyroidism

A

1 Dry blood spots

2 Cord serum

60
Q

Measured for neonatal hypothyroidism

61
Q

Most sensitive test for the diagnosis of congenital hypothyroidism

A

Elevated TSH

62
Q

Cause of false-positive results for neonatal hypothyroidism

A

Low T4 levels (occur in both premature infants and those with congenital absence of TBG)

63
Q

It carries a high false-positive rate for neonatal hypothyroidism

A

Measurement of only T4

64
Q

Increases during pregnancy which elevates the TBG concentration and results in an increase in the total T4 and T3 reference range to approximately 1.5 times the nonpregnancy upper reference level by 16 weeks gestation

A

Estrogen production

65
Q

Gestational transient thyrotoxicosis

A

Syndrome when the increase in FT4 reaches supranormal levels and is prolonged

66
Q

Serum HCG as a physiologic variable

A

1 Increase in T3 and FT4: fertility

2 Fall in serum TSH: first trimester

67
Q

Causes an increase in total T4 and total T3

68
Q

Suppresses TSH

69
Q

Propanolol

A

Suppresses conversion of T4 to T3

70
Q

Amiodarone

A

Causes both hypothyroidism and hyperthyroidism

71
Q

First test to be requested for thyroid disorders

72
Q

Best achieved by immunoassay of plasma and serum samples, such as enzyme immunoassay or fluorescent immunoassay

A

Analysis of thyroxine

73
Q

Test for total thyroxine

74
Q

EMIT: Added to the patient sample along with enzyme-labeled thyroxine (labeled with glucose-6-phosphate dehydrogenase)

A

Antibody that has specificity to thyroxine

75
Q

EMIT: Inhibits the enzyme activity by interfering with the enzyme’s active site for the substrate

A

Binding of the antibody to the enzyme-labeled thyroxine

76
Q

EMIT: Wavelength

77
Q

EMIT: Directly proportional to the concentration of thyroxine in the patient sample

A

Rate of product formation

78
Q

EMIT: Catalyzes substrate (glucose-6-phosphate dehydrogenase) to form product and NADH

A

Leftover unbound enzyme-labeled thyroxine

79
Q

T or F. If the patient sample contains a low amount of thyroxine, there will be a low amount of leftover enzyme-labeled thyroxine to react with the substrate and form the product. When there is low concentration of patient hormone, the majority of the antibody has been bound with the enzyme-labeled thyroxine.

80
Q

Thyroxine reference range: T4 total
Adult female
Adult male

A
  1. 5-11.0 mcg/dl

4. 6-10.5 mcg/dl

81
Q

EMIT: Specimen

A

1 Serum
2 Dried whole blood but heparinized
3 EDTA plasma

Clinical Chemistry Lecture 3 (9 decks)

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